During integrated circuit manufacture and processing, a silicon wafer may be exposed to metallic contaminants such as iron, nickel, zinc, chromium, and the like that may ultimately degrade the final product yield, performance, or reliability. This contamination may occur through contact with stainless steel wafer handlers and tools, diffusion of metallic substances from heater coils or lamps in high temperature processing chambers, or sputter debris dislodging from plasma chamber walls.
Gettering is a term that refers generally to any mechanism by which contaminating impurities, typically transition metals, are removed from sensitive semiconductor device regions and entrapped in other relatively benign domains of the wafer. Gettering is described in Wolf and Tauber, Silicon Processing in the VLSI Era, Vol. 1, 1986, Lattice Press, pp 61-70.
Gettering typically proceeds in three steps: 1) release of a contaminating element from its originating stable state and locale in the wafer into solid solution in the semiconductor crystal lattice; 2) diffusion of the contaminant through the crystal away from sensitive device structures or areas where susceptible structures are ultimately to be formed; and 3) capture of the contaminants by extended defects such as dislocations or precipitates at a position far enough away from devices to avoid interference with their operation and stable enough to prevent future liberation or discharge into the wafer during ensuing thermal, chemical and plasma treatments.
There are two basic categories of gettering mechanisms: extrinsic, or external, and intrinsic, or internal. These categories are discussed in U.S. Pat. No. 4,608,096 to Hill, the disclosure of which is incorporated herein by reference.
Extrinsic gettering entails the use of external means (usually on the wafer back surface) to create damage or stress in the silicon lattice, leading to the creation of extended defects capable of mobile metal capture. Examples of extrinsic gettering approaches include: diffused backside phosphorus or arsenic doping to tie up nickel, gold, iron, copper, etc., and mechanical or physical backside wafer damage produced by abrasion, grooving, sandblasting, laser deformation, ion implantation, polysilicon deposition, etc.
Intrinsic gettering is typically accomplished by the localized capture of impurities at extended defects that exist within the bulk material of the silicon wafer, for example, a Czochralski grown monocrystalline wafer containing interstitial oxygen (5-25 ppma). Intrinsic gettering usually involves the supersaturation of a region or zone of the silicon wafer with oxygen that will separate from solid solution and form clusters of silicon dioxide during thermal treatment. The stresses resulting from the agglomerate clusters cause stacking faults and dislocation loops that are capable of trapping impurities. To be effective, the clusters must be formed in the bulk of the wafer away from active device sites. Oxygen levels above the precipitation threshold must therefore be avoided in regions where active devices will be later be formed and permanently reside.
Various approaches have been taken in the past to provide gettering regions in a bonded semiconductor-on insulator substrate. For example, in U.S. Pat. No. 5,063,113 to Wada, the disclosure of which is incorporated herein by reference, defects for gettering sites are induced in a semiconductor layer by thermal treatment. However the defects are distributed vertically throughout the layer, extending even to the exterior surface, where they may adversely affect bonding of the layer with other layers. In addition, grinding and polishing of the layer results in substantial loss of the gettering sites.
U.S. Pat. No. 5,229,305 to Baker, the disclosure of which is incorporated herein by reference, describes a method for implanting boron, argon, krypton or, preferably, oxygen ions into a polished surface of a semiconductor layer, followed by thermal treatment, to create gettering sites in the layer. The polished surface of the semiconductor layer is then bonded to a handle substrate. Baker does not show implanting silicon ions into the silicon substrate. To create a high density gettering zone, Baker requires a large implant dose. Such a dose of non-semiconductor ions would alter the electrical characteristics of the semiconductor substrate.
Commonly employed gettering techniques are inadequate for use with many desired semiconductor devices. For example, the formation of gettering sites by treatment of the back surface of a device is generally unsuited for application to semiconductor-on-insulator structures. Also, defects that provide gettering sites are often generated in an indiscriminate, scattered fashion throughout the wafer and may thus adversely affect the performance of a device subsequently formed in the wafer.
There is a need for a method for making a bonded semiconductor-on-insulator substrate for integrated circuits that includes a high quality semiconductor device wafer having a smooth surface to promote its bonding integrity to a handle wafer and containing a well-defined, restricted intrinsic gettering zone close to but not detrimentally overlapping sites of devices that are particularly susceptible to metal contamination. There is a further need for a semiconductor substrate whose electrical characteristics are substantially unchanged by the formation of a gettering zone therein. The present invention meets these needs.